We’ve heard about the solder paste “graping” defect, but the same oxidation challenge occurs in other solder forms as well, such as solder washer preforms used to attach small connectors.

Solder graping is a defect that arose as a result of small solder paste deposits and higher (than were used in the past) reflow temperatures. The peak reflow temperature for SnPb alloys were just over 200°C, but. with SnAgCu alloys, that peak is as high as 260°C. New solder pastes have been developed that address this defect by utilizing flux chemistries that function as an oxidation barrier.

But what about other solder forms?

I was recently working with a customer who was evaluating solder preforms for a connector application. These solder washers (outside diameter of 0.025”), like the small solder paste deposits, had a large surface area with a tendency to oxidize on their heated journey to melting. These washers were made of SnAgCu, so the peak temperature required of them was no different than it would be for Pb-Free solder paste.

I found that the same 1°C ramp that causes graping of solder paste can do so for preforms as well. The difference though, is that the non-molten portion of solder takes the shape of its un-reflowed form, that of a small washer in this case, as opposed to the “grapes” of aggregated solder powder.

Fortunately, the issue can be addressed in 3 ways, and I encouraged my customer in this case to process their assembly using the combination:

Adjust the reflow profile.Ed Briggs' recommendations work well here. Further, use a fast ramp to peak. I tested the reflow on a hot plate set to 250°C; this improved the wetting. Preforms are unique from solder paste in that they are solid metal, as opposed to a mixture of metal and flux. They do not benefit from the same heating controls that solder paste requires.

Reflow in nitrogen. By purging out the environmental oxygen during the reflow process, the solder preform will not oxidize.

In the second image here, it is evident the improvement these changes made in terms of the spread and coalescence of the solder preforms. Note that the addition of tacky flux left an amber-colored no-clean residue, however, this can easily be washed away using a mild solvent.

Solders, as a class, are "interesting" metals. And the properties of indium-containing solders are exceptionally interesting. Indium’s (and indium's alloys') physical and mechanical properties are unique when compared to the metallic elements and alloys typically examined.

To put this into context, a metallurgist from a customer company called me because, after looking over our table of solder alloy properties, he claimed our data couldn’t possibly be correct! After a detailed conversation, I understood the nature of his concern. His background was not in solder materials, and the shear strength data for indium (890PSI) is exceeded by its tensile strength (273PSI). This "interesting" situation prompted further questioning. These numbers are, however, accurate.

The graph on right numerically depicts the shear nature of this material. Over a test area of approximately 0.5 square inches, a soldered interface that was sheared at a rate of 1mm/minute to fracture extended 1.6mm before yielding. This extension is indicative of the putty-like nature of pure indium. As expected, The load at yield roughly matched the shear strength cited above for the bulk material because the yield location in this assembly was through the bulk material, rather than along the intermetallic edge.

More extensive information on the physical constants of indium can be found in this application note.

Two categories of solder are available to choose from when the in-service environment for a device reaches above 125°C either in continuous operation or thermal cycling accelerated life testing. These categories are those comprised primarily of lead, and those of gold. From the electronics industry’s drive to eliminate lead, many manufacturers who have traditionally used lead solders are treading cautiously, looking now at the gold solders, primarily at Indalloy 182 (80Au20Sn).

The most common concern regarding this switch relates to the strength of AuSn, which is much higher than the lead solders. The degree that this should be of concern however, should be realized within the scope of the application.

For instance, review this case scenario:

Indalloy 159 (90Pb10Sn) was used in a device for years to adhere high temperature sensors to a calibration probe that is slowly cycled in operation from 350K (~75°C) to 500K (~225°C). The solder joins a nickel and gold plated Kovar™, or platinum or platinum coated, nickel lead to a tinned copper lead. The solder joint is not placed under tension or shocked.

Considering the high temperature solder options in this scenario, the AuSn would be mechanically preferred.

Why?

Well, tin-bearing soft solders will leach gold from gold metallizations during soldering, creating a brittle Au-Sn intermetallic layer within the solder joint. The more gold available, the more consumed, and the greater the thickness of the resultant intermetallic layer. The brittle nature of this layer, situated intimately next to the relatively soft PbSn solder layer, creates differential stresses that promote crack propagation upon thermal cycling.

AuSn was not considered previously because the engineers were familiar with its hardness and, given the cracking failure described using a softer solder, they did not anticipate improvement. It was a pleasant surprise to them to find that switching to a lead-free solder would not sacrifice the quality of their device. AuSn is a brittle alloy but, unlike the description above, no differential stresses are involved.

Note: Eutectic gold solders have been used for many years to solder nickel plated Kovar™ lids to high reliability ceramic packages and have a good history of fatigue performance.

A new patent has been published for a liquid metal gallium plus carbon nanotube thermal interface that may capture considerably more of the thermal conductivity benefit claimed by researchers of carbon nanotubes (CNTs) than has been possible outside of a laboratory setting previously.

Admittedly, I am embarrassed not to have come up with this thermal management concept myself! I was so close, and yet so far away. Previously, documented back to 2008, I described how conductive metals are in their liquid state. Those atoms get moving and shaking, and the heat passes right through!

I also balked at those touting their CNTs…. The technology wasn’t there! Carbon Nanotubes cannot be affordably grown on substrates to make a commercially-affordable thermal interface material, and if you grow them, cut them down, and try to re-apply them, they are no longer oriented to directly connect substrate layers, and the resultant interfacial resistance between tubes adds up fast.

Well, this patent reveals that Foxconn put 2 and 2 together to make what I’m gambling to be an outright GREAT Thermal Interface Material!!! This patent calls for carbon nanotubes suspended in liquid metal. That liquid metal fills in all the tube-to-tube gaps, and as fast as the heat comes in, it moves on out!

So, for those of you with carbon nanotubes, Indium has the liquid metal, numerous types in fact, to suspend them and make a great thermal interface material.

I love learning new technology, just wish that I had the clarity to have pursued something so obvious myself!

Only a few solder alloys have become common, industry-wide, among solar module assemblers, and those can be pared down into three categories:

BiSn alloys (58Bi42Sn, 57Bi42Sn1Ag)

SnPb alloys (63Sn37Pb, 62Sn36Pb2Ag)

SnAg alloys (96Sn4Ag)

The wetting attributes and reliability of SnPb alloys have long made them an attractive selection, however, in green technologies such as these, Pb-free material selections are preferred. Jim Hisert previously discussed the benefits of BiSn as a low temperature Pb-free solder alternative for tabbing solar cells, so I will touch on the SnAg alternative.

Tin-Silver Solder (SnAg)SnAg has become the most widely used Pb-free solder alloy, particularly in tabbing ribbon designed for cell interconnection. Historically, its melting temperature (221°C) made it an obvious replacement for processes previously running SnPb solders.

In designs where step soldering is necessary (however uncommon in back end solar module assembly), SnAg can be used as the step previous to soldering with Sn63 or similar Pb-Free solder (albeit carefully since the second soldering temperature is quite near 221C).

While SnAg eutectic solder is a desirable composition for electronic component soldering, for instance, power semiconductors, recent studies using this alloy for stringing solar modules have indicated that the other common alloys listed for this application are easier to work with and better designed to meet the needs of this solar assembly application. SnAg does have a high melting temperature, and the preferred fluxes for module assembly are not yet optimized for this solder composition.

Regardless, SnAg has its benefits. When a solder that melts somewhat above the melting point of a “standard” solder alloy is needed, and it must be Pb-free, this is it!! Check it out!

Solder fluxes have not traditionally been used with AuSn, AuGe, or AuSi eutectic solder, because their peak reflow temperatures were very close to or above the flux activation range. An average flux activates at approximately 125°C and is not recommended for temperatures in excess of 350°C. Although AuSn solder melts at 280°C, peak reflow temperatures are recommended to be >300°C, nearing the maximum suggested temperature of flux. AuGe and AuSi alloys melt at 356°C and 363°C respectively, which exceed the documented flux activation range.

Since these alloys contain ≥80% gold and are resistant to oxidation, flux is not always necessary. Other methods have conventionally replaced the flux function, such as mechanical scrubbing, or forming gas purging. If these technologies are not available, or assembly speed is priority though, a flux may be required.

So I, along with my fellow engineer, Brandon Judd, sought out to test some of our best fluxes with Au alloys at these high temperatures. The result: Not all, but a few of these fluxes work extremely well up to temperatures as high as 450°C!!!

The reflow profiles used tested the extreme abilities of our fluxes:

·Peak temperature 410°C

·Nitrogen Purge

·Time above liquidus: 137 seconds

·80AuSn solder preforms 0.249” square x 0.002”

Some fluxes did what we expected- they charred and burned. They simply were not designed for this environment.

Others, such as our TacFlux010® were very resilient at these temperatures.

For more information about these test results, please contact myself or Brandon.

These mating surfaces are assumed to be bonded via a solder bond. That begs the question, “Can AlSiC be soldered directly? Is the aluminum metal filler free and in great enough density to provide for metallic surface bonding?”

In order to be sure of this answer, I went to another AlSiC materials expert, Tom Sleasman, business manager at Rogers Corporation. Rogers Corporation offers a new, special AlSiC variety with an aluminum skin (AlSic-D3), which I thought would have the best solderability of any of these materials, if that was even possible.

Unfortunately, Tom’s response was, “We do not recommend soldering directly to the AlSiC for the same reasons as to aluminum. Our process yields a part with an Aluminum rich skin on the outside of the part and in all cases [where the AlSiC is soldered] I am aware of utilizing a nickel plating or copper coating process to provide a solderable surface.”

I cannot speak directly to soldering these new aluminum skin AlSiC materials, however soldering to aluminum is a difficult process, which cannot be accomplished without the appropriate flux. That flux is typically acid-based and offered only as a stand-alone product, not as a solder paste or preform flux coating.

Based on Tom’s remarks, I conclude that the best way to solder AlSiC is to have it pre-coated with an oxide-resistant metal, such as nickel or copper. Regular AlSiC substrates cannot be soldered without this surface treatment.

I have previously discussed various reasons why thermal considerations in a device cannot be an afterthought. There are various methods for handling the thermal needs of a device before it becomes a problem. One of my well-known colleagues, Ross Wilcoxon, Principal Mechanical Engineer at Rockwell Collins, knows a great deal about these. His article, “a spreadsheet based matrix solution for a thermal resistance network: part 1” was highlighted in Electronics Cooling Fall 2010, and in it he discusses a method using Excel to do thermal resistance modeling.

My inquisitive nature couldn’t let the article stand on its own. I had to tack on a few more questions. Ross was accommodating enough to help me out.

[Amanda Hartnett] Why is it important to characterize the thermal resistance of each material used in a device?

[Ross Wilcoxon] It may not be - sometimes the best lesson learned from a resistance network analysis (or any modeling effort for that matter) is determining which things are really important to the final results (component temperatures, reliabilty, etc.) and which things aren't. For example, if the aluminum chassis in a system plays a critical part in the overall thermal resistance and has a large temperature gradient, then it is pretty important to know what alloy it is so that you can better estimate its thermal conductivity. On the other hand, if the chassis is pretty much uniform in temperature, then knowing exactly what its thermal conductivity is probably doesn't matter so much.

[Amanda Hartnett] What information is needed from the material vendors in order to complete a resistance network analysis?

[Ross Wilcoxon] Obviously, thermal conductivity is a good start and if you are doing a transient analysis (I plan to talk about that in part 3 of the series that I would like to do for Electronics Cooling), information on specific heat and density are pretty important. For interface materials, the overall interface resistance is needed more than the thermal conductivity. In many cases, it would be really nice to have data not only for nominal values but also some indication of uncertainty. The resistances in a thermal network can be calculated using best or worst case numbers as easily as they can with nominal material properties. It is pretty easy to switch between these values within the spreadsheet and it is a good way to get a feel for how important knowing the precise value really is by looking at how varying between best and worst values impact the overall temperatures. Also, I have done a few spreadsheet based Monte Carlo simulations for getting my hands around the cumulative effects of uncertainty in things like thermal gap fillers and a thermal test stand. For that type of analysis, you have to have some understanding of the uncertainty as well as the nominal values.

[Amanda Hartnett] Could a model like this be used to characterize the effect of degradation in a single layer?

[Ross Wilcoxon] I guess I'm not sure exactly what you mean on this. If the effect of the single layer (I suppose you mean a thermal interface material) is accounted for in the thermal resistance calculation, sure - you can just apply a factor in the equation to account for something like voiding to say that the effective thermal conductivity of the interface material decreases by X% to assess how much that impacts the overall effect. I suppose it just comes down to how complicated you get in converting material and geometry parameters into thermal resistance.

[Amanda Hartnett] In a typical cooling solution, have you found that one boundary was more critical than another?

[Ross Wilcoxon] In a lot of cases, the thermal battle is lost in the first mm of the thermal path (the interface between a component and whatever it is attached to - I bet you like that answer, huh?!) but in a lot of our systems the choke point is in the last mm (moving the heat from the system to the surroundings). One of the big benefits of network resistance analysis is the fact that you can very easily adjust the resistances, including these boundary conditions, by just changing a couple cells in the spreadsheet. This can give a good feel for which parameters are the most critical and what needs to be better understood. For example, in the next article for Electronics Cooling (assuming that I get it written), I plan to talk a bit about an analysis that I did for some of our equipment going into a missile pod along with equipment from a number of other suppliers. At the time of the analysis, we didn't know certain details about things like the surface finish to which we were attaching our module, the specific alloys in the missile pod, etc. Having a quick-look analysis tool helped us determine which unknowns were really critical for the thermal analysis and we could concentrate in chasing down that information.

-----------------------------------

Ross – Thank you for your time and for sharing your knowledge and experience!

I’ve received numerous questions about using gallium liquid metal alloys, so thought I’d present some of my answers for all. The customer questions are in black, and my responses in red.

Indium’s product data sheet, "Indalloy Metals Liquid at Room Temperature" mentions that "any liquid metal will wet another clean metal surface". Can you please elaborate on the conditions that make such wetting possible?

As far as wetting, gallium alloys (Indalloy 46L, Indalloy 51E, Indalloy 60, etc.), coat nearly any organic, ceramic, or metal surface. It is difficult to come up with materials used in packaging and processing these alloys that come clean of the alloy after use. The physics of this affinity is unknown, however the low melting point and surface tension are the source/consequence.

Many metals will form some alloy with the gallium, but the solubility in gallium is limited. The gallium wets the surface and forms a solid gallium-alloy layer, which then acts as a diffusion barrier. In the case of aluminum, gallium forms an amalgam which ends up consuming a large volume of aluminum before a stable solid layer.

Do you assume an oxygen atmosphere (gallium oxide does wet most surfaces)? Or, to the opposite, do you refer to the wettability of plasma-clean metal surfaces?

This wetting behavior relates to surfaces in air or vacuum. Surfaces do not have to be atomically clean. Gallium oxide does form as a film over the surface of a pool of these alloys. However, it does not diminish the wetting behavior.

Liquid metals are typically offered in a syringe and various needle gauges can be screwed to the tip to adjust the dispensed droplet size. For dispensing thin layers of liquid metal, such as for a thermal interface, we recommend PVA selective coating equipment.

We are interfacing our thermoelectrics with both steel and aluminum and the hottest point will be around 600°C. Can liquid metals be used for this?

InGa alloys are selected typically because they remain liquid at all times, negating any contact resistance between substrates. The typical application for these has temperatures up to approximately 100°C or slightly higher. At these temperatures we have noted some aluminum corrosion by the gallium, which will only become worsened by your elevated temperature.

There are other alloys which do not contain gallium and have only a slightly higher temperature, such as the Bi/In alloys. At 70°C, the eutectic composition of this alloy will also be liquid in phase, however is much less reactive with your bonding metals, and therefore may be better suited.

Over the past few years, I have been forced to come to terms with my current (and ever-growing) knowledge of “things”. It is easy to become overwhelmed by the engineering veterans who have minds swollen with information following their decades of professional exposure. The key is to admit what you do not know so that you can learn from their explanation, and then chime in when your own expertise fits and appears helpful.

I am not an expert of all “things.” In fact, I am so terrible at Jeopardy that my husband has developed his own character skit of me “playing” the show (and losing miserably) from my living room couch as he listens in humor from the kitchen. I am an expert in some things though, and these are what I highlight in my thermal blog.

This is why I am so excited to see a fellow engineer starting his own thermal blog!! Highlighting his expertise in the thermal benefits of AlSiC, Mark Occhionero describes the usefulness of this material in various applications such as hermetic packages, baseplates for power modules, and lids for microprocessors.

AlSiC materials have great thermal conductivity and the ability to shift CTE based on the amount of SiC filler used. This allows for CTE matching of lids to dies, and packages. With CTE-matched substrates, high reliability soldering for bonding and thermal interconnections is a breeze!!

At some point we have all had experiences which convinced us that metals have a high thermal conductivity. It may have been the hot spoon you left in your coffee after stirring in a little cream and sugar, or the hot door handle you grabbed on a simmering hot summer day when climbing into that now-vintage car of yours. In fact, the high thermal conductivity of metal can even account for the ability to get your tongue stuck to a metal pole in the cold of winter (or the metal screen door while waiting for the school bus as was my childhood experience). We generally understand the phenomena of metals to have a high thermal conductivity to be true, however what is the basic science behind the high thermal conductivity of metal?

The article describes metal generically as positive ions within a “communal sea of their valence electrons”, together providing a net neutral charge. The image above depicts this arrangement. A metal is unique because unlike non-metallics which are viewed as highly organized lattices, valence electrons of metal atoms are not strongly held by the nucleus and are highly mobile. These mobile electrons transfer electric charge as well as heat across the metallic structure. This freedom of the valence electrons accounts for the high thermal conductivity in metals. At ambient temperatures, metals are attributed with high conductance, however an additional rise in thermal conductivity is found as environmental temperatures rise. This activity can be explained using the principles explained in the Wiedemann-Franz Law.

In electronics packaging, there are many materials to choose from which will provide various thermal dissipation outcomes. Metallic materials are generally preferred for high power devices due to their high thermal conductivity, lending them for adoption in heat sinks, heat spreaders, baseplates, and even thermal interface materials as Indium is most familiar with.

Conductive epoxy is a common material choice for bonding components, especially if the assembly process is temperature-sensitive. Tin-based solder paste or preforms with flux are preferred Pb-free bonding materials; however, conductive epoxies arguably provide advantages over these traditional solder assembly materials.

It has been my experience that these advantages are perceived in the absence of an awareness of the full solder assembly materials product offering. Specialty solders can provide the same advantages as conductive epoxies and then some.

Some claimed advantages to conductive epoxies include:

·RoHS-compliance

·Ease of assembly

·No-clean

·Low cure temperatures

Low-temperature solders such as 58Bi42Sn and 52In48Sn are specialty low-temperature solders which have these same properties including processing temperatures below 150ºC. Both of the referenced alloys are Pb-free, can-be used with no-clean fluxes and are assembled using the traditional solder assembly techniques.

It would seem a toss-up between whether to use a conductive epoxy or specialty solder to assemble temperature-sensitive components except that there are additional advantages to a soldered assembly as compared with an epoxy-assembly. These include:

I have been asked on numerous occasions to calculate the potential for galvanic corrosion between metals. Most times, when I am approached with this, the concern stems from an application in which the bonding metals will be mated in a corrosive environment, such as a salt solution.

When the potential is to be calculated for two elemental metals bonding, the potential for galvanic corrosion is simple to calculate. Simply look up the anodic potential difference between the two metals under the galvanic series in a general chemistry handbook and if the value is less than 0.15V (the maximum recommended for a salt solution), galvanic corrosion should not be a concern. For normal environments, such as storage in warehouses or non-temperature and humidity controlled environments there should not be more than 0.25 V difference in the Anodic Index. For controlled environments, such that are temperature and humidity controlled, 0.50 V can be tolerated.

This value is much more difficult to calculate, however if the bonding metals are alloys rather than elemental metals.

For instance, I cannot easily supply the anodic potential difference between 80Au20Sn and a pure Au plating to prove that it is less that 0.15V. This is because I cannot calculate the anodic potential theoretically for the AuSn alloy. Data is readily available for pure metals, but the potential for individual solder alloys must be determined experimentally because the voltage potential is not linear and as you begin to add a second metal to a pure metal, the rate of voltage change is different between different alloys.

For this exact situation, I can speak practically however. We have tested gold plated Kovar lids for corrosion that were sealed to semiconductor packages that had a gold seal ring using a preform of AuSn. They were tested for corrosion in a salt spray chamber per MIL STD 883. Corrosion, when it occurred, always was on the lid where the porous gold allowed underlying nickel corrosion. There was never an instance of corrosion at the Au/Sn and Au interface region.

Generically, a phase change material is one which will store or release energy when it changes phase from solid to liquid or liquid to solid. According to this generic classification, there are 4 general categories of phase change materials.

These phase change material categories are not all-encompassing, however. Other materials such as metals, eutectic or not, are used as phase change materials for their thermal energy storage and removal abilities.

Nearly all soft solders classify as phase change materials according to their melting temperature. According to Maurice J. Marongiu from MJM Engineering, who conducted a webinar on phase change materials, the melting temperature of a typical phase change material is between 0-250ºC. Solders officially may melt at higher temperatures, such as the AuGe eutectic alloy which melts at 356ºC, however the majority of solders used melt below 250ºC.

Phase change materials are a common occurrence in the world of thermal interfaces for electronics. Here, tighter commonalities between phase change materials can be found. For instance, the phase change temperature for these thermal interface materials is within the range of a common TIM junction temperature, which is typically lower than 100ºC. For this reason, when we consider a metal interface to be a phase change material in this industry, it is an alloy or material which changes phase below 100ºC.

When implementing a metal or non-metal phase change material into a thermal interface, there are some design considerations to be made:

Phase change materials are applied as solid pads. At room temperature they are firm and available with specific dimensions which make them easy to handle. Consistent application should be inherent.

Phase change materials each change phase at a unique temperature. The appropriate phase change material engineered for an application will have a phase change temperature reached within the normal operating cycle of the device.

Phase change materials are designed to turn liquid in operation. The liquid phase of these materials will have a distinct viscosity. Depending on the material, clamping pressure and assembly orientation, the molten material may leak. Proper precautions should be taken to prevent material leakage, especially toward active electrical components if the phase change material is electrically conductive.

When reservoirs are created to contain a phase change material, these reservoirs must accommodate the liquid phase of the phase change material as well as the solid phase. As a phase change material changes from solid to liquid there is an increase in the material volume. If the phase change material expands and fractures the reservoir, this will lead to leaks and the eventual failure of the electronic device as the thermal interface becomes backfilled with air.

Soldering through-hole connectors can be a tedious task. Connector Specifier recently highlighted an article by principal engineer, Paul Socha discussing how connected preforms can be used to streamline the soldering process.

Many through-hole connectors can be hand soldered successfully using solid or flux cored wire. Others are more difficult for reasons including long pins, thick boards, or difficult-to-reach connections. Connected (integrated) preforms relieve these issues by supplying custom preforms to match the application.

To read the article and to learn more about connected preforms and how to design them for your needs, visit Connector Specifier.

Eutectic Gold Tin (AuSn) with a composition of 80Au20Sn is a unique material. This particular alloy of gold tin (AuSn) is considered a solder because it has a melting temperature of 280ºC, which is lower than the 350ºC transition temperature into braze materials. Still, there are some similarities between this solder alloy and braze alloys. The most obvious is the hardness of the gold tin (AuSn) alloy. With a tensile strength of 40,000PSI, this solder is much more rigid than the tin solders most are familiar with. The strength is more closely compared to the silver brazes which melt above 500ºC.

With that strength has come some unique manufacturing difficulties. For many years, one obstacle for implementation of gold tin (AuSn) as a solder preform or wire, was its availability in thin forms or fine diameters. The gold tin (AuSn) is extremely hard and it became brittle as it was handled through manufacturing and would crack if it was pressed too thin or fine.

Luckily, in the 40+ years since eutectic gold tin (AuSn) was first used in electronics manufacturing, processing techniques have come a long way. Today, gold tin (AuSn) solder can be made into dimensions much smaller than the soft solders, allowing it to be used in applications which require the highest level of precision.

Typical dimensions and tolerances of gold tin (AuSn) can be found in the below chart.

This chart as well as more detail on gold tin (AuSn) applications are available in the paper titled, “Process and Reliability Advantages of AuSn Eutectic Die-Attach,” presented at IMAPS 2009.

Materials to be used in packaging of high power semiconductor devices are often chosen by their coefficient of thermal expansion, or CTE. For instance, substrates such as AlSiC, Molybdenum, and Tungsten are chosen to mimic the coefficient of thermal expansion (CTE) values of the materials they will be attached to so as they expand and contract, the substrates do so in tandem, minimizing the mechanical stresses at the interfaces between these areas, or their CTE mismatch.

The coefficient of thermal expansion (CTE) of indium does not match many materials, yet it is chosen commonly as a solder thermal interface material between substrates with as dissimilar substrate properties as silicon and copper.

How can indium bond together silicon with a CTE of 2.6PPM/ºC and copper with a CTE of approximately 17 PPM/ºC, then undergo years of thermal and power cycling, and not show degradation of thermal performance?

The answer is in the strength and malleability of indium. Indium is the softest metal which is stable in air. Although the CTE of indium is 29 PPM/ºC, the tensile strength of indium is 273PSI, which is very soft, and the shear strength of indium is 890PSI, which is significantly higher. In an application where indium is soldered to a back-side metallized die and a copper integrated heat spreader, there is significant CTE mismatch.

However, assuming the interfaces of these solder joints is sound with minimal voids, the bulk indium will bend and stretch along with the contraction of substrates and will not crack.

We've had some questions about using liquid metal or indium alloys liquid at room temperature containing indium and gallium, and their reaction to stainless steel.

Many scientists are actively searching for a stable, thermally conductive liquid material to replace NaK. Our data sheet on the liquid indium/gallium metals indicates that these are corrosive to many metals. Will the liquid metals which are indium and gallium (Indalloy 51E) react with stainless steel?

The gallium alloys have been tested as replacements for NaK liquid metals for reactor vessels and have performed acceptably up to ~600°C. They are also used in switches with immersed stainless steel electrodes (the make/break element is usually tantalum to reduced arcing damage). I don't think you will see any wear in stainless components at the temperatures used for the application of Indalloy 51E.

Long Answer:

Any liquid metal will wet another clean metal surface. In practical interactions, thin layers of oxygen, nitrogen, and carbon are often sufficient to prevent the wetting (the science of surface chemistry and fluxes has been developed to overcome these issues).

The gallium alloys are able to break down this surface layer without any flux. This puts the base metal and the liquid alloy in intimate contact. If the base metal has appreciable solubility in gallium, it will dissolve. The rate is limited by the mass transport from the solid into the liquid alloy. Temperature, turbulence, and solubility of gallium in the metal affect the rate.

If the temperature of the exposure is hot enough, the stainless steel elements will begin to dissolve into the alloy. Turbulence at the surface of the alloy breaks down the diffusion boundary layer—like stirring to dissolve sugar in coffee. Up to ~400-600°C iron, chromium and nickel in stainless steel are essentially mutually-insoluble with gallium (as is the case with the refractory elements tantalum, tungsten, molybdenum, zirconium, titanium, etc). Above this temperature, the elements in stainless steel begin to dissolve in the gallium.

Aluminum is in the same Periodic Group as gallium and the two have wide solution ranges--~20% gallium is soluble in solid aluminum and ~1% aluminum dissolves in gallium at its melting point. When the gallium alloys come in contact with aluminum, essentially no diffusion barrier prevents the gallium from displacing aluminum in the solid and turning into an Al-Ga amalgam.

0402 or 0603 solder preforms deposited in paste for pin in paste assembly

Pin-in-paste is the technique of intrusive soldering through-hole components to a circuit board using reflow soldering.Typically, solder paste can be printed and this will provide enough solder for complete barrel fill.

Occasionally, more solder is needed than can be printed, and solder preforms can be used.

Phil Zarrow and Jim Hall of ITM Consulting have a recorded discussion forum called "Board Talk" in which they discuss common board assembly issues and solutions. One of the topics they have discussed is called, "Through Hole Reflow - Pin in Paste."Here, they discuss the typical techniques used for soldering through-hole components to a circuit board.These include printing solder paste around the through-hole in the board, inserting the through-hole connector, and reflowing the assembly in a convection oven.

They mention that one way to apply more solder is to overprint the solder paste.For many applications, this is an excellent technique.The overprinted solder paste will wick into the barrel during the reflow stage.This is a natural process because the over-printed area is typically a non-solderable solder mask surface outside the plated through-hole annular ring.The solder de-wets from the solder mask and surface tension pulls it into the pool of solder, onto the annular ring, and down the plated barrel of the board.

Occasionally, overprinting solder paste still does not provide enough solder to fill the through-hole.On these occasions, a good alternative is solder preforms.

There are a couple types of solder preforms to consider.The first is a standard 0402 or 0603 size preform supplied in carrier tape.These preforms are 100% metal compared with solder paste, which is only approximately 50% metal by volume.To incorporate preforms, print solder past around the through-hole.Place a preform just outside the annular ring, but touching the solder paste.Just like overprinting, the solder preform supplies extra metal, but this time, the metal from the solder preform will wick down the through-hole barrel and provide much more solder than the overprint was able to.The flux in the solder paste will be enough to remove oxides from the preform, so no additional flux is needed.

A second type of preform to consider for the most difficult-to-solder connectors are integrated preforms.These preforms are custom-designed for each connector type and include an array of washers.One washer will fit over every pin.Flux is applied and the connector is inserted.The assembly is reflowed in a convection oven, and more solder is available to fill the barrel than could be supplied using solder paste at all.This is the most manual technique, however, may be the only option to achieve a complete barrel fill without re-designing the board.